In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Two relevant parts to the function of the edge or tip of a cutting tool include the side or flank face, where metal separation occurs under high pressure, and the top or rake face, where the metal chip flows under low pressure and high shear stress, deforming internally and rubbing against the tool. Chip flow 2 is schematically illustrated in FIG. 1. Due to low pressure at the rake face 4 and finite adhesion chip-to-rake surface, the chip deforms at high strain rate both within the chip and at the rake surface leading to friction heat 6. Under high pressure at the flank face 8, the workpiece acts brittle and separates via cracking with less shear deformation.
On the flank face, friction heat is less than on the rake face. Therefore, on the flank face, wear is mainly by abrasive wear, for example, by hard grits in the metal scratching against the hard flank surface of the tool. On the rake face, wear is mainly by adhesive or thermal-chemical mechanisms including diffusion, alloy formation, and reaction forming new softer phases than the original tool material, or by spalling, chipping and delamination, in which pieces much larger than the grain size of the tool break away.
Flank wear is important since flank wear directly impacts the metal cutting operation. As flank wear proceeds, the tool progressively removes less material. Manual intervention will generally be required to correct the machine path or machined parts of incorrect dimension will be made. This ruins productivity of the machining operation. Thermochemical wear on the rake face is not as critical as it does not affect the quality of the machined parts directly. The rake surface carries the chip away. However, when wear on the rake face proceeds too far, the tip may become geometrically undersupported, bend too much under normal chip forces and break off. As wear on the rake face increases, chip contact increases as does the strain rate within the chip and chip-tool contact area, all of which can accelerate friction heat generation.
One important problem with chip formation and adhesion at the rake face is that friction heat generated there conducts to the flank, where it reduces tool hardness and amplifies abrasive wear. Tool hardness, being derived from compression, is compromised by heat, which produces expansion. When friction heat is sufficiently great, the flank can overheat and suffer thermochemical wear, which can be manifested in features such as gouging, chipping and notching. When this happens, it can be difficult to discern wear pattern differences between the rake and the flank surfaces. Advanced thermochemical wear at the flank can mar the fresh workpiece surfaces, ruining machined part quality, perhaps long before part dimensions are impaired by flank wear or the tool is in jeopardy of breaking. For hard steel turning, surface quality due to flank overheating is typically the most common mode of premature tool life that can waste the advantage of hard and/or superhard tool materials.
Therefore, methods of reducing friction heat and/or reducing the transfer of friction heat from the rake race to the flank face contribute to increased performance of the cutting tool or tool insert.
Overheating is a problem of all tool materials, but particularly a problem with thermally labile, metastable superabrasive tools based on diamond and cubic boron nitride (cBN). The hardness of superabrasive materials is keenly sensitive to temperature.
Friction heat is much worse when cutting soft, non-lubricated metals at high speed with high depth-of-cut (doc) and feed rate. However, the requirement of flank hardness is considerably less than when cutting hard, non-lubricated steels. Friction heat is much reduced by addition of lubricants to the metal.
The general solution to the heat problem for superabrasive tools is to add ceramic to the tool material, improving heat tolerance. However, adding ceramic to superabrasive materials can make the material defective because bonding ceramic to superabrasive materials is not easily accomplished. It is preferred to put ceramic on the rake surface and not in the material of the tool body per se, to allow the ceramic to be effective in carrying away the chip without defecting or reducing the hardness of the superabrasive material itself.
Coatings can act as anti-friction and/or thermal barriers, reducing the production of friction heat and the conduction of friction heat to the flank. Coatings reduce adhesive friction by putting a chip-repelling anti-adhesive material layer between the deforming metal chip and any metal or metal-like component(s) of the tool material. This reduces adhesion as well as internal shear deformation within the chip itself. Coatings may also prevent oxidation of the tool material to softer oxides. Coatings comprising majority covalent ceramics may act as poor heat conductors, reducing heat conduction to the flank.
Coatings are applied as a final step to finish-ground tools and can be placed on all surfaces, including the chamfer 10, the flank 8 and the rake 4 faces, examples of which are shown in FIG. 1. On the flank 8, the coating, typically being softer than the tool material, abrasively wears away quickly exposing the harder and more wear resistant tool material underneath. On the rake 4, the coating abrasively wears slowly due to a lower pressure.
Typically the coating spalls, chips, cracks, delaminates or flakes off due to severe heating, adhesion failure and associated thermochemical wear processes. Typically, once the coating is gone, the rake and flank overheat and fail. The usual manner to improve coating life is to reduce the thickness of the coating and to increase compression on the coating. However, a thin coating has even less wear resistance.
Coatings on cutting tools are normally applied by low pressure gas phase physical vapor deposition (PVD) or chemical vapor deposition (CVD) processes and are limited to <0.020 mm thickness due to increasing stress, grain growth and delamination. Thin coatings limit thermal insulation and wear away faster than thick coatings.
A better coating will be thicker, more inert, non-adhesive to the workpiece, hard (low abrasive wear rate), tough to both mechanical (vibration, impacts) and thermal tensile strains (expand/contract) so as not to crack, and well adhered, both within the bulk of the coating and at the coating interface with the tool material, so as not to delaminate. Achieving all this with conventional PVD and CVD gas phase deposition methods is challenging. PVD cannot make thick coatings effectively. Due to low PVD temperatures, the grains within the PVD films are not well developed, and thus are of lower hardness and bonding strength. CVD coatings produced up to 1000° C. are better crystallized than PVD. However, CVD coatings >0.020 mm thick are generally not successful and generally cause tool embrittlement.